Two-stage valve arrangements for activating a piston of a hydraulic actuator are known from EP 2 234 135 B1 and DE 10 2009 053 901 B3, respectively, wherein the hydraulic actuator is provided for activating a high-voltage circuit breaker. The valve stages include a pilot stage and a main stage.
The valve arrangement from EP 2 234 135 B1 includes a 3/2-port valve as a pilot valve and two 2/2-port valves as main valves, whereas in the valve arrangement from DE 10 2009 053 901 B3 two 2/2-port valves are also present as main valves but the function of the 3/2-port pilot valve is performed by two 2/2-port valves as pilot valves.
The valve arrangement from EP 2 234 135 B1, together with the piston/cylinder arrangement controlled by the valve arrangement and with the high-voltage circuit breaker which is to be activated, is shown in FIG. 1. The valve arrangement and piston/cylinder arrangement are jointly part of a hydraulic actuator for activating the high-voltage circuit breaker.
In a first position of the pilot valve 11, the port from a high-pressure tank 9, via the main valve 2, to the space 10 situated in the piston/cylinder arrangement 12 is opened. i.e. the fluid which is at high pressure is supplied to the space 10 above the piston 6 so that the high-voltage circuit breaker 7 is closed. The fluid can be a hydraulic oil. In a second position of the pilot valve 11, the space 10 is connected to a low-pressure tank 8 via the main valve 1, i.e. the space 10 above the piston 6 is depressurized, as a result of which the piston 6 moves backwards and opens the high-pressure circuit breaker 7.
The valve arrangement from DE 10 2009 053 901 B3 can be seen in FIG. 2, together with the piston/cylinder arrangement 12 of the hydraulic actuator and with the circuit breaker 7 which is to be activated. Identical elements to those in FIG. 1 carry the same reference numerals, and the fundamental mode of operation of the valve arrangement is the same as described above for FIG. 1.
The pilot valves 3 and 4 in FIG. 2 take the form of NC valves which are electrically activated and have a spring return. Owing to the spring return which is required for technical reasons, after switching the situation always occurs in which an enclosed oil volume, which is responsible for the correct positioning of the valves 1 and 2 of the main stage for the period until the next switching event, exists in the pilot region, i.e. in the hydraulic region of the pilot stage.
The 2/2-port valves 3 and 4 of the pilot stage can have leaks. Depending on the switching position, the internal leakage at the pilot valves 3 and 4 can now result in an undesired build-up or reduction of pressure, which could compromise the correct positioning of the main valves 1 and 2.
This should be prevented as otherwise the main stage could malfunction and hence the actuator and the high-voltage circuit breaker could malfunction.
This issue is addressed in DE 10 2009 053 901 B3 by incorporating a small aperture 5 between the pilot region, i.e. the hydraulic region in which the oil is enclosed, and the main region, i.e. the region which actuates the main piston 6. Specifically, the aperture 5 is situated between the outlet side X, connected to the control inlets of the main valves 1, 2, of the pilot valves 3, 4, and the outlet side Z, connected to the piston/cylinder arrangement 12, of the main valves 1, 2.
Such a connection between the pilot region and the main region is, however, undesirable during switching as the aim is precisely that the two regions operate hydraulically independently of each other (cf also FIG. 1).
In the static situation, when there is therefore no switching, the aperture does, however, help that any leaks which may possibly exist are compensated, wherein the fact is exploited that, in the static situation, the same pressure always prevails in the pilot region and in the main control region, and that the main control region is not sealed and hence capable of compensating the leaks which have occurred. As there are very low volume flows in the static situation, the aperture functions like an open connection between the main and the pilot control region, i.e. a flow of oil from the pilot region caused by a leak can pass via the aperture into the main control region and be discharged there.
The aperture is designed to be as small as possible so that this open connection does not result in an adverse effect on the functioning of the valve arrangement during switching, i.e. in a dynamic situation. The effect of significant and widely varying differences in pressure prevailing between the main and the pilot region, as a result of which very large volume flows occur at the main valves, is thus exploited. Owing to the very small size of the outlet opening of the aperture in comparison with the respective known opened valve and because everything happens extremely quickly in the dynamic situation, the aperture then acts like a separator between the main and the pilot region.
Accordingly, the aperture fundamentally separates the main and the pilot region in the dynamic situation and provides a low-loss connection between the main and the pilot region in the static situation.
The aperture therefore represents in each case a compromise between “as small as possible” for the dynamic situation and “as large as possible” for the static situation. It has now been proved in practical tests that this compromise may not be sufficient in individual cases.